An anode is a critical component in a cathodic protection system, which is used to prevent corrosion in metal structures such as pipelines, storage tanks, and ship hulls. The anode is a sacrificial or impressed-current electrode that is strategically placed in an electrolyte (such as soil or water) and connected to the metal structure requiring protection. In sacrificial anode systems, the anode is made of a more reactive metal, like zinc or magnesium, which corrodes preferentially, thereby protecting the less reactive metal structure (the cathode) by supplying it with electrons. In impressed-current systems, anodes are typically made of durable materials like graphite or mixed metal oxides and are powered by an external electrical source to drive the protective current. By releasing electrons into the electrolyte, the anode ensures the protected structure remains cathodic, preventing oxidative corrosion. The anode’s design, material, and placement are tailored to the specific environment and structure to ensure effective, long-term corrosion protection.
Anode Materials
- Galvanic (Sacrificial) Anode - Galvanic anodes are made from metals that are more electrochemically active (less noble) than the metal they are protecting. When connected to the structure and immersed in an electrolyte (like soil or water), the galvanic anode corrodes preferentially, sacrificing itself to protect the structure. These systems don't require an external power source.
- Magnesium (Mg) - Magnesium has the most negative (active) potential among common galvanic anodes, making it suitable for environments with higher resistivity, such as onshore buried pipelines and certain freshwater applications. Primarily used for buried structures in soils and occasionally in freshwater tanks. Consumes relatively quickly, especially in low-resistivity environments, and can generate hydrogen gas.
- Zinc (Zn) - Zinc is less active than magnesium but more active than steel. It's effective in environments with lower resistivity, like seawater and brackish water, and low-resistivity soils. Widely used for marine pipelines, ship hulls, and in freshwater ballast tanks. Zinc ribbon anodes are also used for mitigating induced AC on buried pipelines. Performance can be negatively affected by temperatures above 60°C, where its polarity relative to steel can change.
- Aluminum (Al) - Aluminum alloys, often with small additions of zinc, indium, or mercury, are highly effective in seawater due to their good current output and high ampere-hour capacity. Commonly used for offshore structures, ship hulls, and marine pipelines. Requires specific alloying elements to prevent passivation (formation of a protective oxide layer that stops current flow) in some environments.
- Impressed Current Anode - Impressed current cathodic protection (ICCP) systems use an external DC power source (rectifier) to drive current from the anodes to the structure. This allows for a more controlled and higher current output, making them suitable for larger or more complex structures. Impressed current anodes are made from materials that are designed to be corrosion resistant and long lasting, as they do not "sacrifice" themselves in the same way galvanic anodes do.
- Mixed Metal Oxide (MMO) Coated Titanium - These anodes consist of a titanium substrate coated with a thin layer of mixed metal oxides (e.g., iridium oxide, ruthenium oxide). They are highly durable, dimensionally stable, and provide excellent current distribution. Widely used in various environments, including seawater, brackish water, freshwater, carbon backfill, and concrete. They are a popular choice for long pipelines, ship hulls, and steel in concrete. Cost-effective in the long run due to their longevity and high current output per unit area.
- High Silicon Cast Iron (HSCI) - HSCI anodes are a traditional and robust choice. They offer good resistance to corrosion in various environments. Commonly used in groundbeds for pipelines and storage tanks, and in some marine applications. Can be brittle and heavy, and their consumption rate can be higher than MMO anodes in some conditions.
- Graphite - Graphite is a carbon-based material that is electrically conductive and resistant to many corrosive environments. Often used in deep well groundbeds and sometimes in freshwater or soil applications. Can be consumed over time and may require backfill to maintain good electrical contact.
- Platinum (and Platinum-Clad Niobium/Titanium) - Platinum is highly corrosion-resistant and has excellent electrochemical properties. It's often clad onto a titanium or niobium substrate to reduce cost while retaining performance. Used in specialized applications where high current density, long life, and chemical inertness are critical, such as in aggressive chemical environments or specific marine applications. High cost limits their widespread use, but their performance justifies it in certain scenarios.
- Scrap Steel - While not as efficient or long-lasting as other ICCP anode materials, scrap steel can be used as a low-cost option, particularly in certain soil environments. More common in some regions as a cost-effective solution for temporary or less critical applications. High consumption rate and requires significant material volume for effective protection.
